As easy-to-access and easy-to-produce hydrocarbon resources are depleted, there is an increased demand for more advanced recovery procedures. One such procedure is steam assisted gravity drainage (SAGD), a procedure that utilizes steam in conjunction with two spaced apart wellbores. Specifically, SAGD addresses the mobility problem of heavy oil in a formation through the injection of high pressure, high temperature steam into the formation. This high pressure, high temperature steam reduces the viscosity of the heavy oil in order to enhance extraction. The injection of steam into the formation occurs from a first wellbore (injector) that is drilled above and parallel to a second wellbore (producer). As the viscosity of the heavy oil in the formation around the first wellbore is reduced, the heavy oil drains into the lower second wellbore, from which the oil is extracted. Commonly, the two wellbores are drilled at a distance of only a few meters from one other. The placement of the injector wellbore needs to be achieved with very small margin in distance. If the injector wellbore is positioned too close to the producer wellbore, the producing well would be exposed to very high pressure and temperature. If the injector wellbore is positioned too far from the producer wellbore, the efficiency of the SAGD process is reduced.
It is well known that traditional surveying techniques, often referred to as “ranging”, utilized to evaluate the distance between two wellbores suffer from a widening cone of uncertainty as the wellbores become longer, making it more difficult to achieve the precision in placement that is required in SAGD applications. Electromagnetic (EM) systems and methods have been employed in ranging to determine direction and distance between two wellbores.
In EM ranging systems, one of the wellbores is cased in a conductive metal (typically steel). This wellbore is typically referred to as the “target” wellbore and usually represents the SAGD injector wellbore. In any event, a current is applied to the target wellbore casing by a low-frequency current source. Currents flow along the wellbore casing and leak into the formation. The currents result in an EM field around the target wellbore. The EM fields from the currents on the target wellbore casing are measured using an electromagnetic field sensor system disposed in the other wellbore, which is typically the wellbore in the process of being drilled. This second wellbore usually represents the SAGD producer wellbore. Although it may be deployed on wireline, tubing or the like, in many cases, the electromagnetic field sensor system is carried by a drill string and represents a logging-while-drilling (“LWD”) system.
The distance and direction from the LWD device to the target wellbore can be determined if the magnitude of the current on the target wellbore is known. However, while it is generally understood that the magnitude of current decreases along the length of the target wellbore and that current will leak into the formation, typically, the actual magnitude of the current, and particularly the distribution of the current along the target wellbore, is unknown. Thus, it is common practice to simply estimate the magnitude of the current in a target wellbore in order to yield ranging results. Otherwise, without knowing the current, the ratio of EM fields and/or their gradients can approximate the distance and direction from the LWD device to the target well. To improve upon LWD ranging, it would be advantageous to know the magnitude and distribution of current along the target wellbore.